Make the Heka build less fragile

Currently the Heka image build sometimes fails with the
following error:

	2016-09-07 18:08:09.219 2868 DEBUG fuel_ccp.build [-] heka: [91mCMake Error at /tmp/heka/cmake/message_proto.cmake:2 (message):
    Google protocol buffers 'protoc' must be installed, message.proto has been modified and needs to be regenerated.

This error is related to the "make" process trying to
re-generate "message.pb.go" using "protoc" which is not
present. And re-generating "message.pb.go" is not necessary
and shouldn't be done.

This commit attempts to fix the issue by touch'ing the
"message.pb.go" file to make sure the "make" process will
not attempt to re-generate it.

Change-Id: I91f12a99c813be99ba5a24ff65ca786eb97fea0c

docker/heka/install-heka.sh

@@ -19,6 +19,10 @@ echo "Get and build Heka..."
 cd /tmp
 git clone -b dev --single-branch https://github.com/mozilla-services/heka
 cd heka
+touch message/message.pb.go  # make sure message/message.pb.go has a date
+                             # more recent than message/message.proto, to
+                             # prevent make from attempting to re-generate
+                             # message.pb.go
 source build.sh  # changes GOPATH to /tmp/heka/build/heka and builds Heka
 install -vD /tmp/heka/build/heka/bin/* /usr/local/bin/
 cp -rp /tmp/heka/build/heka/lib/lib* /usr/lib/